Update plexus to 2.1.1

This requires pinning Findbugs version to 3.0.4 as 3.0.0 fails with the exception:
"Unable to parse configuration of mojo org.codehaus.mojo:findbugs-maven-plugin:3.0.0:findbugs for parameter pluginArtifacts:
Cannot assign configuration entry 'pluginArtifacts' with value
'${plugin.artifacts}' of type java.util.Collections.UnmodifiableRandomAccessList to property of type java.util.ArrayList"

Author: wendigo

pom.xml

@@ -55,7 +55,7 @@
     <properties>
         <project.build.targetJdk>1.8</project.build.targetJdk>
         <dep.maven-api.version>3.0</dep.maven-api.version>
-        <dep.plexus.version>1.5.5</dep.plexus.version>
+        <dep.plexus.version>2.1.1</dep.plexus.version>
         <dep.plexus-utils.version>2.0.4</dep.plexus-utils.version>
         <dep.plugin.plugin.version>3.4</dep.plugin.plugin.version>
     </properties>
@@ -200,6 +200,7 @@
             <plugin>
                 <groupId>org.codehaus.mojo</groupId>
                 <artifactId>findbugs-maven-plugin</artifactId>
+                <version>3.0.4</version>
                 <configuration>
                     <excludeFilterFile>${project.basedir}/src/build/findbugsExcludeFilter.xml</excludeFilterFile>
                 </configuration>